1. Field of the Invention
This invention relates to the field of combustible gas detectors or sensors. Uniquely, the detector has multi-level sensitivity circuitry.
2. Description of the Related Art
Various devices are well known for combustible gas detectors used to detect the presence of combustible gases such as those found in car engines. Typical circuits are configured to include at least one sensing element that may be a wire having a catalytic coating. The sensing element was used as one of four legs of a wheatstone bridge circuit. The other three legs consisted of two resistors and a compensator element. The compensator element was identical to the sensing element except that it did not bear a catalytic coating.
A current or voltage was applied to the bridge circuit to heat the surface of the catalytic coating affixed to the sensing element. Since the resistance values of the other three legs of the bridge were known, the resistance in the sensing element could be determined as the current or voltage was passed through the bridge.
When the sensing element was exposed to a combustible gas, such as hydrocarbon, a reaction would occur, increasing the temperature of the sensing element. As the temperature of the sensing element increased, the resistance of the element increased. Accordingly, the current or voltage passing through the element decreased. By comparing the resistance level of the sensing element to the resistance level of the compensator element, the presence of a combustible gas could be detected. Since the amount of gas caused a nearly linear increase or decrease in the resistance of the sensing element, the quantity of the gas could be accurately determined by calibrating the change in resistance. This is the basic operating principle of a catalytic combustible gas sensor.
3. Related Art
Examples of patents related to the present invention are as follows, and each patent is herein incorporated by reference for the supporting teachings:
U.S. Pat. No. 5,526,280 is a method and system for gas detection.
U.S. Pat. No. 5,517,182 is a method for CO detection and its apparatus.
U.S. Pat. No. 5,379,630 is a thermal conductivity detector.
U.S. Pat. No. 5,365,216 is a catalyst monitoring using ego sensors.
U.S. Pat. No. 5,363,091 is a catalyst monitoring using ego sensors.
U.S. Pat. No. 5,297,419 is a linearizing gas analyzer.
U.S. Pat. No. 5,225,786 is a combustible gas sensor.
U.S. Pat. No. 5,087,574 is a fluid component detection method with feedback.
U.S. Pat. No. 5,057,436 is a method and apparatus for detecting toxic gases.
U.S. Pat. No. 5,055,269, is a temperature limited catalytic gas detector apparatus.
U.S. Pat. No. 4,938,928 is a gas sensor.
U.S. Pat. No. 4,847,783 is a gas sensing instrument.
U.S. Pat. No. 4,829,810 is a filament drive circuit.
U.S. Pat. No. 4,818,977, is a combustible gas detector having temperature stabilization capability.
U.S. Pat. No. 4,804,632 is a method for detecting combustible gases and device therefor.
U.S. Pat. No. 4,574,264 is a thin film oxygen sensor with microheater.
U.S. Pat. No. 4,533,520 is a circuit for constant temperature operation of a catalytic combustible gas detector.
The foregoing patents reflect the state of the art of which the applicant is aware and are tendered with the view toward discharging applicants' acknowledged duty of candor in disclosing information that may be pertinent in the examination of this application. It is respectfully stipulated, however, that none of these patents teach or render obvious, singly or when considered in combination, applicant's claimed invention.
4. Related Problem
Hydrocarbon sensors must measure gas concentrations in an exhaust stream environment that can range from a relatively low level to a very high level of gas concentrations, ie. 100 ppm to over 1000 ppm. The problem occurs when a designer must choose to build either a high or low level of gas concentration sensor. If a sensor is built to sense high concentrations, the resolution may typically be around 50 ppm, which is not very useful for sensing low concentrations of gas. If a low concentration sensor is built, the resolution may be as low as 1 ppm, but the sensor will saturate and not be able to sense high concentrations of gas. Therefore, a need exists for a sensor that can accommodate many different ranges of gas concentrations accurately.
This and other problems will be solved by the preferred embodiments of the invention. A review of the specification, drawings, and claims will more clearly teach a skilled artisan of other problems solved by the preferred embodiments.